1. Field of the Invention
The present invention relates to a communication device mounted on, for example, a global positioning system (GPS) on a portable terminal such as a mobile phone.
2. Description of the Related Art
In a GPS system for measuring the position of a mobile body utilizing satellites (GPS satellites), a basic function of the GPS receiver is to receive signals from four or more GPS satellites, calculate the position of the GPS receiver from the received signal, and inform that to users.
The GPS receiver demodulates a signal from a GPS satellite to acquire orbital data of the GPS satellite, and derives its own three-dimensional position from information of the GPS satellite orbit and time and delay time of the received signal by simultaneous equations.
The reason why four GPS satellites giving the received signal are required is that there is an error between the time inside the GPS receiver and the time in the satellites and that error must be eliminated.
That is to say, the GPS receiver can calculate the positioning by receiving the radio transmitted from the GPS satellites.
In the case that radio from four or more satellites can be received, by dividing the deference from the transmission time of each satellite signal and the receiving time of the GPS receiver by the velocity of light the distance to the satellite is obtained, from the distance of the GPS receiver to each GPS satellite the position of the GPS receiver and the present time can be obtained.
Further, by using a reference frequency that is had inside of the GPS receiver, the received frequency from each satellite is obtained, and the velocity of the GPS receiver and an error of the reference frequency can be obtained from the received frequency (refer to “improved edition basic of GPS survey”, Atsushi Tsuchiya and Hiromichi Tsuji work, Japanese association of surveyors).
Moreover, inside of the GPS receiver, the GPS signal is acquired by using the above reference frequency that a crystal oscillator generates, and by tuning it to the frequency of the radio transmitted from the GPS satellites, and the received frequency from the GPS satellites is obtained.
A general GPS system, as shown in FIG. 1, has an antenna 1 receiving the radio of not illustrated GPS satellites, a crystal oscillator 2 generating the reference frequency used by a GPS receiver, a GPS receiver acquiring and calculating the positioning by using a GPS signal received by the antenna 1 and a frequency generated by the crystal oscillator 2, and a host CPU controlling the GPS receiver.
This general GPS system supplies the electric power to the GPS receiver 3 always from a power source for obtaining the position-finding position at high speed, and makes the GPS receiver store an error value of the reference oscillation frequency of the crystal oscillator 2. Moreover, the GPS receiver 3 uses a signal received by the antenna 1 received and frequency generated by the crystal oscillator 2, makes the frequency as reference frequency, acquires the GPS signal and calculates positioning, and the host CPU 4 obtains the result from the GPS receiver 3.
A general process of the GPS system will be explained further concretely.
In the case of a consumer GPS receiver, a positioning computation is carried out by receiving a spread spectrum signal radio referred to as the L1 band or C/A (coarse acquisition or clear and acquisition) code from a GPS satellite (Navstar).
The C/A code is a signal obtained by the binary phase shift keying (BPSK) modulating a carrier wave (hereinafter referred to as a “carrier”) having a frequency of 1575.42 MHz by a signal obtained by spreading data of 50 bps by a code of a pseudorandom noise (PN) sequence having a transmission signal rate (chip rate) of 1.023 MHz and a code length of 1023, for example, the Gold code.
In this case, since the code length is 1023, the C/A code is formed as a code that a PN sequence code is repeated using 1023 chips as one cycle (=1 millisecond (msec)) as shown in FIG. 2A.
The PN sequence code of this C/A code is different for every GPS satellite, but is composed so that which GPS satellite uses which PN sequence code can be detected by the GPS receiver in advance.
Moreover, the navigation message mentioned above enables the GPS receiver to turn out from which GPS satellite signals can be received at the position and the point of the time.
Therefore, in the case of for example three- dimensional positioning, the GPS receiver receives radios from four or more GPS satellites which can be acquired at the position and the point of the time, despreads the spectrum, and performs the positioning computation to find its own position.
Then, as shown in FIG. 2B, one bit of satellite signal data is transmitted as 20 cycles of the PN sequence code, that is to say, 20 milliseconds. Namely, data transmission rate is 50 bps.
In 1023 chips of one cycle of the PN sequence code are inverted between when the bit is “1” and when the bit is “0”.
As shown in FIG. 2C, in the GPS, one word is formed by 30 bits (600 milliseconds). Further, as shown in FIG. 2D, one sub-frame (6 seconds) is formed by 10 words.
As shown in FIG. 2E, the word at the header of one sub-frame has a preamble always regarded as a bit pattern even if data is updated inserted to it, after this preamble data is transmitted.
Further, one frame (30 seconds) is formed by five sub-frames. In addition, the navigation message is transmitted by data units of this one frame. The first three sub-frames in this one frame data from information inherent in the satellite referred to as ephemeris information. This information includes parameters for finding the orbit of the satellite and transmission time of the signal from the satellite.
All GPS satellites have atomic clocks and use common time information, the transmission time of the signal from the GPS satellite is a one second unit of the atomic clock. Moreover, the PN sequence code of the GPS satellite is generated as a code in synchronization with the atomic clock.
The orbital information in the ephemeris information is updated every several hours, however, until the information is updated, it is the same information.
However, by holding the orbital information of the ephemeris information in the memory of the GPS receiver, the same information can be precisely used for several hours.
Note that the transmission time of the signal from the GPS satellite is updated every one second.
The navigation message of the remaining two sub- frames in one frame data is information commonly transmitted from all the GPS satellites referred to as almanac information.
This almanac information needs 25 frames in order to acquire all information, and it is composed of approximate position information of each GPS satellite and information indicating which GPS satellite can be available and so on. This almanac information is updated every several months, however, until the information is updated, it is the same information.
However, by holding the almanac information in the memory of the GPS receiver, the same information can be used at high accuracy for several months.
For receiving the GPS satellite signal and obtaining the above data, first, after removing the carrier, the PN sequence code (hereinafter PN sequence code will be referred to as PN code) the same as the C/A code used in the GPS satellite to be received prepared in the GPS receiver is used to acquire, the signal from the GPS satellite and spread the spectrum.
When the phase synchronization with the C/A code and the despread is performed, the bit is detected and it becomes possible to acquire the navigation message including time information from the GPS satellite signal.
The acquisition of the signal from the GPS satellite is performed by phase synchronization search of the C/A code, in this phase synchronization search, the correlation between the PN code of the GPS receiver and the PN code of the received signal from the GPS satellite is detected. For example, when the correlation value of the result of the correlation detection is larger than preset value, it is judged that both are synchronized. When it is judged that synchronization has not been established, any kind of synchronization technique is used to control the phase of the PN code of the GPS receiver to synchronize with the PN code of the received signal.
Incidentally, as mentioned above, the GPS satellite signal is a signal that carrier is BPSK-modulated by a signal that data is spread by a spread code. Therefore, in order that the GPS receiver receives the GPS satellite signal, it is necessary to establish synchronization of not only the spread code but the carrier and the data, however, synchronization of the spread code and the carrier cannot be independently performed.
Further, in the GPS receiver, the received signal is converted carrier frequency of that to an intermediate frequency within several MHz, and it is general that the synchronization detection process mentioned above is performed by an intermediate frequency signal.
The carrier in the intermediate frequency signal includes a frequency error mainly due to a Doppler shift according to the velocity of the GPS satellite and a frequency error of a local oscillator generated inside the GPS receiver when the received signal is converted to an intermediate frequency signal.
Therefore, due to these frequency error factors, the carrier frequency in the intermediate frequency signal is unknown, so a frequency search for that becomes necessary.
Moreover, since a synchronization point (synchronization phase) in one cycle of the spread code depends on positional relationship between the GPS receiver and the GPS satellite so is unknown, some kind of synchronization technique becomes necessary.
The GPS receiver uses a synchronization technique by a frequency search for the carrier and a sliding correlator+DLL (Delay Locked Loop)+Costs loop.
This will be explained below.
The clock driving a generator of the PN code of the GPS receiver is generally a clock obtained by dividing an oscillation signal of a reference frequency oscillator provided in the GPS receiver.
As this reference frequency oscillator, a high accuracy crystal oscillator is used, and a local oscillation signal used for converting the received signal from the GPS satellite to an intermediate frequency signal is generated from the output of this reference oscillator.
FIG. 3 is a view for explaining this frequency search.
As shown in FIG. 3, when the frequency of the clock signal for driving the generator of the PN code of the GPS receiver is a certain frequency f1, the phase able to establish the synchronization is made to be detected by phase synchronization search of the PN code, that is to say, by sequentially shifting the phase of the PN code by each one chip, detecting correlation between the GPS received signal and the PN signal in each chip phase and detecting the peak value of correlation.
When the frequency of the clock signal is f1, and there is no synchronized phase in all phase search of 1023 chips does not exist, for example the frequency division ratio for the reference frequency oscillator is changed, the frequency of the drive clock signal is changed to f2, and the phase search of 1023 chips are performed in the same way.
As shown in FIG. 3, this is repeated by stepwise changing the frequency of the drive clock signal.
The above operation comprises the frequency search.
Moreover, by this frequency search, when frequency of the drive clock signal regarded to be possible to be synchronized is detected, the final phase synchronization of the PN code is carried out at the clock frequency.
However, the above-mentioned technique as a synchronization method is unsuitable for fast synchronization in principle, in an actual receiver, it would be necessary to search for the synchronization point in parallel by forming multi-channels for compensating the unsuitableness. Moreover, if the time is required for synchronization of the spread code and the carrier as mentioned above, the response of the GPS receiver becomes slow, and inconvenience is arisen for usual use.
Consequently, as for the phase synchronization of the spread code, without using a method of sliding correlation as mentioned above, a technique that the phase synchronization by a digital matched filter using fast Fourier transform (FFT) processing is realized by the improvement of the capability of the hardware such as a digital signal processor (DSP).
Incidentally, in a reference frequency oscillator applied to the GPS system, an oscillation frequency is basically fixed, however, since a frequency generated by a crystal oscillator has an error due to the temperature change, the secular change and so on, the frequency range for searching the radio from the GPS satellite is needed to be defined in a wide range, therefore in a conventional GPS system, there is a disadvantage that the time is required for acquiring the signal from the GPS satellite.
Moreover, in the conventional GPS system, for obtaining the positioning computation at high speed, it becomes necessary to supply electric power continually for the GPS receiver 3. In addition, in the case that the electric power is supplied only when the positioning result is required, since the frequency error value of the crystal oscillator cannot be held, the frequency range including this error is searched, as a result, the time was required for the positioning computation.